This invention relates generally to constant current circuits and, more particularly, is directed to a microprocessor controlled constant current circuit.
Circuits which generate a constant current are well known in the art. However, such circuitry is relatively complex. In addition, such known circuitry is continuously operative, thereby having a relatively large power requirement.
In many instances, however, it is necessary to provide an isolated output current for use with microprocessor-based instrumentation, which circuitry is accurate, compact and inexpensive. For example, for use with microprocessor-based process pH and conductivity monitoring instrumentation, such as that sold by Beckman Industrial Corporation of Cedar Grove, N.J., it is necessary to provide an isolated output current in the range of 4-20 mA. Since the microprocessor controls many other operations of the apparatus, it cannot be used to continuously monitor and adjust the required output current, without detracting from the other operations it must perform. Therefore, to prevent this result, the circuitry must be made more complex.
U.S. Pat. No. 3,646,650 discloses a circuit in which, when no signal is received by the receiver, a transistor is normally turned ON which passes a low impedance to the gate of a SCR, thereby maintaining the SCR in an OFF state. Under such conditions, the output of a bridge rectifier charges a capacitor through a diode. A Zener diode regulates the power supplied to the capacitor. When a proper signal is received by the receiver, the transistor is turned OFF so that the SCR is controlled by the voltage across a resistor. In this mode, the SCR is operated to provide a constant current. In addition, the SCR produces a high impedance in its OFF condition. See also, U.S. Pat. Nos. 3,867,641 and 3,512,000.